The invention relates to an image forming apparatus such as a laser printer and a digital copying machine, and an optical beam scanning device that can be used in the image forming apparatus.
For example, a color image forming apparatus of plural drum type is constituted so that a plurality of images are created on photoconductor drums, the images are overlapped with one another on a recording medium, and one image is formed. The apparatus utilizes a plurality of image forming sections corresponding to separated color components, and an optical beam scanning device (laser exposing device). The optical beam scanning device provides image data corresponding to the color components, namely, a plurality of laser beams to the image forming sections.
In such a color image forming apparatus, it is necessary to suitably establish a positional relationship of the images on the photoconductor drums, particularly the positional relationship in a sub-scanning direction in order to realize a color image without color shift (color registration error). A lot of optical elements, however, intervene between light sources and the photoconductor drums, and the optical elements are influenced by ambient temperature and humidity, thereby occasionally causing color shift.
In order to suppress the color shift in the sub-scanning direction (displacement between the color component images), the method disclosed in U.S. Pat. No. 6,337,757 is proposed.
In this method, pre-deflection optical systems for respective color components, which are provided between light sources for the color components and a deflector shared by the color components, are provided with a hybrid cylinder lens. The hybrid cylinder lens is composed of a glass cylinder lens having a positive power in the sub-scanning direction through which light beams transmit with eccentricity and tilt, and a plastic cylinder lens having a negative power in the sub-scanning direction. As a result, displacement in the sub-scanning direction due to a change in the ambient temperature is suppressed for each color component.
In the above conventional method, however, the plastic cylinder lens should be provided for each color component (when the color components are yellow, magenta, cyan and black, four plastic cylinder lenses), thereby making the apparatus complicated and expensive.
When the hybrid cylinder lens is moved to an optical axial direction for focus adjustment that focuses the beams on image surfaces (surfaces of the photoconductor drums), the hybrid cylinder lens allows the light beams to transmit with eccentricity and tilt. For this reason, emitting positions on the image surfaces (beam positions) are changed.